Description of the Related Art
The present invention relates to orthopedic implants for placement at least in part into bone, or for placement at least in part between adjacent bones or bone portions of a human skeleton. The orthopedic implants of the present invention have a specialized surface for engaging bone.
Vital to the functioning of such orthopedic implants is their ability to remain properly located within bone after installation. In U.S. Pat. Nos. 5,593,409 and 5,609,635, Michelson described the use of surface roughenings, such as knurling or ratcheting on the opposed upper and lower vertebral body engaging surfaces of interbody spinal fusion implants. Knurling has been particularly beneficial for increasing the grip of the implant surface to the bone of the adjacent vertebral bodies in a rather uniform manner without a directional bias. Orthopedic implants inserted at least in part into bone, have a propensity to move in a particular direction, which is opposite to their path of insertion, because this is the path of least resistance. Such propensity to move is further increased when the implant has an overall configuration that is tapered along at least a portion of its length such that the bone engaging surfaces of the implant are in angular relationship to each otherand are spaced further apart at the implant's trailing end than at the implant's leading end. In such circumstances where it is desirable then to gain stability in resistance to a particular direction of movement of the orthopedic implant, the use of a plurality of forward facing ratchetings on the implant's bone engaging surfaces has been preferable to the previously described knurling for that purpose.
The term “ratcheting” as used herein is defined as a plurality of angular teeth or ridges or protrusions projecting from the surface of an implant to resist motion of the implant at least in one direction. The phrase “forward facing ratchetings” as used herein is defined as a ratcheting having at least one forward facing facet having a length greater than a rearward facing facet and an angle from the implant surface from which the forward facing facet arises that is less steep than the angle of the rearward facet. On an implant surface, forward facing ratchetings facilitate the insertion of the implant in one direction and after insertion, resisting movement of the implant in a direction opposite to the direction of insertion. An example of forward facing ratchetings of the prior art is shown in partial fragmentary view in FIGS. 24A and 24B, generally referred to by the reference numeral 50.
Knurled surfaces of the related art provide some stability in all directions, but lack the ability to resist a particular direction of motion preferentially. The above-described ratcheted surface best resists motion in a particular direction. There exists a need for an improved orthopedic implant surface configuration, wherein the bone engaging surfaces of the implant are configured to be resistant to implant movement in all directions, and preferentially in particularly one direction.